1. Field of the Invention
The present invention relates to a signal processing method for a fiber optic interferometric sensor. More particularly, the present invention relates to a counting signal processing method for a fiber optic interferometric sensor.
2. Description of the Related Art
Single mode fiber optic interferometric sensors such as the Mach-Zehnder interferometers, the Sagnac interferometers and the Michelson interferometers are very sensitive sensors that can be used to detect changes in a physical field such as acoustic pressure or temperature and produce phase signal. The two-arm fiber optic interferometric sensor, such as the Mach-Zehnder interferometer or the Michelson interferometer, uses a distributed feedback (DFB) laser with narrow linewidth (that is, larger coherence length) as the system light source to reduce the phase noise resulting from the optical path difference between the two fiber arms. The Sagnac interferometer uses a superluminescent diode with wide linewidth (that is, smaller coherence length) as the system light source to reduce the Rayleigh scattering noise resulting from the sensing fibers. The laser beam of the light source passing through a leading fiber is split by an optical coupler and transmits into the sensing fibers. When the laser beam passes through the sensing fiber with a length L, the optical phase delay is ψ=knL, where k is the optical wavenumber in vacuum, n is the refractive index of the fiber core.
The variation in the optical phase delay of the sensing fiber are caused by the strain and the refractive index change due to acoustic pressure. The two-arm fiber optic interferometric sensor uses the difference variations of the optical phase delay between the two sensing fiber arms, the Sagnac interferometer uses the difference variations of the optical phase delay of the clockwise and the counterclockwise directions of the sensing fiber, both can generate an output intensity containing the sensing phase signal. The output intensity of the interferometric sensor is converted to electrical output signal by an optical receiver.
In the conventional technique, the electrical output signal from the fiber optic interferometric sensor must be demodulated by a demodulation circuit so that sensed phase signal can be linearly demodulated. The most commonly used signal demodulation circuits include the active homodyne with DC phase tracking and the passive homodyne demodulation using phase generated carrier etc. These demodulation circuits of the fiber optic interferometric sensor not only increase the system complexity, but also cause a lot of constraints when operating. For example, some of the demodulations need to add PZT phase modulators to the fiber arms. When the sensing fiber arm has a phase modulator, a driving electrical signal is required. Therefore, the sensing terminal of the optical fiber sensing system can no longer be constructed using all passive elements and the optical fiber sensor can only be used in some specific environments. In addition, the signal demodulation circuits have a dynamic range and frequency band limitations. Therefore, when the sensed phase signal contain the large amplitude and the high frequency, the output signal of the demodulation circuit will encounter the saturation and the distortion.
In some specific applications, the demodulation circuit is only necessary to distinguish whether the sensing signal is large enough. In this situation, there is no need to perform complicated linear signal demodulation. For example, when the fiber optic interferometric sensor is applied to an anti-theft security system, the ability to detect the change in the sensed phase signal caused by an intruder is all that is required. There is no practical need for linearly demodulate the sensed phase signal. Furthermore, the transient sensing phase signal with large amplitude and high frequency is induced by an intrusion, hence, if the foregoing linear demodulation circuit is deployed, saturation or distortion problem will occur more frequently and the complexity of the fiber optic interferometric sensor will increase unnecessarily.